1. Field of the Invention
This invention pertains generally to the electrostatic filtration of particulates from a gas stream that contains both particulates and fibers.
More particularly, this invention pertains to corona discharge electrodes for imparting electric charge to particulates in a gas stream that contains both particulates and fibers.
2. State of the Prior Art
The term "particulates" is conventionally used to indicate aerosol particles having a longest dimension less than 100 microns. The term "fibers" is conventionally used to indicate aerosol particles having a length-to-diameter ratio greater than 10, with the length being in the 10.sup.3 to 10.sup.4 micron range.
In many industrial operations (e.g., the manufacture of textiles from cotton, wool or synthetic materials), particulates and fibers become entrained in gas streams such as streams of air for collecting dust and waste fibers in the vicinity of processing machines, or air that is circulated by air conditioning apparatus. In a typical filtration system as known in the prior art for removing particulates and fibers from a gas stream, a mechanical fiber separator is positioned in the gas stream to remove fibers from the gas stream in gross quantity. However, respirable particulates in the gas stream, which tend to be harmful to health, pass through the fiber separator to a significant extent, and remain in the gas stream until removed by some other technique.
As a practical matter, a mechanical fiber separator as used in a typical gas filtration system is considerably less than 100% effective in removing fibers from the gas stream. A typical technique for removing particulates and fibers that have escaped removal from the gas stream by a mechanical separator comprises imparting electric charge to the particulates and fibers downstream of the mechanical separator, and electrostatically attracting the charged particulates and fibers onto a filter medium.
A conventional technique for imparting electric charge to particulates in a gas stream uses a corona discharge electrode, or a plurality of corona discharge electrodes, positioned in the gas stream. Ordinarily, at least some fibers along with the particulates that pass through the fiber separator enter into the corona discharge region and become electrically charged. The gas stream then carries a major portion of the electrically charged fibers along with the electrically charged particulates to an electrostatic filter medium, where the charged fibers and particulates are removed from the gas stream by deposition onto the filter medium.
Corona discharge electrodes as used in imparting electric charge to particulates in the atmosphere of an industrial plant usually comprise fine wires of, e.g., 10-mil outside diameter. It has been found, however, that a significant portion of the electrically charged fibers in the gas stream become entangled on the corona-generating electrodes in the corona discharge region. Such entangled fibers tend to form a fibrous coating on the corona-generating electrodes.
When a coating of high-resistivity fibers builds up on the corona-generating electrodes of a gas filtration system, the voltage needed to maintain a desired rate of ion production in the corona discharge region can increase significantly. Raising the voltage to a level sufficient to maintain ion production in such circumstances, however, can cause sparking with the attendant danger of fire or explosion. Furthermore, the build-up of a fibrous coating on the corona-generating electrodes can significantly impede gas flow in certain types of gas filtration systems.